In the field of rubber industry, especially, automobile components are desired to have advanced function and high performance. A V-ribbed belt in which ribs are provided along a belt longitudinal direction is one of rubber products to be used in such automobile components, and this V-ribbed belt is widely used for, for example, power transmission of auxiliary drive of automobile air compressors, alternators, and the like.
As a production method of a V-ribbed belt, there is known a production method of a rib part (compression rubber layer) in an inverted trapezoidal sectional shape by grinding. Specifically, in a production process of a V-ribbed belt, first of all, a belt sleeve in which respective molding members (e.g., an outside fabric, an unvulcanized rubber sheet, a cord, etc.) are wound around the outer peripheral surface of a cylindrical die and laminated is formed. In general, as for the belt sleeve, a sleeve is formed such that a ground surface (compression rubber layer which forms ribs) is faced on an outer peripheral side, whereas a belt back surface is faced on an inner peripheral side. Subsequently, the belt sleeve is disposed within a vulcanizer in a state where a vulcanizing jacket is put on the outer peripheral side of the belt sleeve, followed by undergoing vulcanization. In vulcanization of the belt sleeve, the vulcanization is conducted in a state where the outer peripheral surface of the belt sleeve is brought into contact with the inner peripheral surface of the vulcanizing jacket, and after the vulcanization, the vulcanizing jacket is removed (released). In addition, breathing (air bleeding) is necessary such that air (air bubble) does not build up in the belt sleeve during the vulcanization. In order to secure the releasability and breathability (air bleeding), there is adopted a method in which vulcanization is conducted in a state where a thick nonwoven fabric is wound on the outer peripheral surface of the sleeve, and after release, the nonwoven fabric is also ground and removed together with a ground site of the compressed rubber (method of grinding the entirety of rib parts).
In recent years, from a viewpoint of cost reduction, grappling with reduction of the material costs by reduction of the grinding amount (waste rubber amount) or by reduction of the belt thickness or the like is made. As for the reduction of the grinding amount (waste rubber amount), there is investigated a method in which the entirety of ribs is not ground, but only V-grooves (only side parts) are ground without grinding tip surfaces of the rib parts (bottom parts in an inverted trapezoidal shape). However, according to this method, the outer peripheral surface of the vulcanized belt sleeve becomes the rib tip surface as it is, and therefore, when a nonwoven fabric is used for the outer peripheral surface, the nonwoven fabric remains in the tip surface of rib part (rib tip surface).
As for the V-ribbed belt having a nonwoven fabric on the rib tip surface, for example, PTL 1 proposes that in order to suppress generation of abnormal sounds generated between the belt and the pulley or wear on the belt surface without using a short fiber-containing rubber, the rib rubber layer is constructed in a structure in which a rubber layer and a nonwoven layer are alternately laminated in a belt thickness direction and discloses a V-ribbed belt having the nonwoven layer on the rib tip surface of the rib rubber layer.
However, in this V-ribbed belt, since the rib rubber layer contains the plural nonwoven fabric layers, the belt becomes rigid, and therefore the bending properties of the belt are worsened (the nonwoven fabric cramps up, thereby disturbing bending) and the durability is worsened (a crack is liable to occur) in belt running. Furthermore, since the rib rubber layer is divided by the nonwoven fabric layer, delamination is liable to occur, too. Moreover, in this patent literature, in order to suppress abnormal sounds generated between the belt and the pulley and to suppress wear of the friction belt surface, the nonwoven fabric layer is introduced in place of a short fiber to be contained in the rubber, and problems in the grinding method are not described. In this patent literature, details of the nonwoven fabric layer, such as a weight per unit area, etc., are not described. In general, in V-ribbed belts having a nonwoven fabric on the rib tip surface, the outward appearance on the nonwoven fabric surface is worsened.
Meanwhile, if the production is conducted without using a nonwoven fabric, not only the releasability and breathability (air bleeding) become insufficient, but also the surface properties of the vulcanizing jacket are transferred onto the sleeve surface (rib tip surface), and for example, in the case of a vulcanizing jacket having a flaw on the surface thereof, the flaw is transferred, whereby the appearance is worsened.
In addition, PTL 2 discloses a V-ribbed belt having a thermoplastic resin layer (layer in a film-like form but not a fibrous form) on the rib tip surface. In this patent literature, it is described that after the thermoplastic resin layer is joined to the rib, the belt is cut and ground in a V-belt profile.
However, even in this V-ribbed belt, not only the breathability (air bleeding) is insufficient, but also the bending properties of the belt are worsened (the thermoplastic resin layer cramps up, thereby disturbing bending), and the durability is worsened (a crack is liable to be produced).